Galactic motion (hijack from where does space end)

[David Levy]

Think of the Galaxies on that balloon as ants. If the surface was sticky and they couldn't move, then they'd simply get further away from each other. None would collide. But, those ants can move. Some, being close enough, detect another ant and move close to discuss Wittgenstein.

 

Simple question:

 

In our Local Group there are three main spiral galaxies:

Milky Way, Andromeda Galaxy and Triangulum Galaxy.

Andromeda is quite close to Triangulum Galaxy, while our distance to Andromeda is significantly longer (2.54 Mly).

Therefore, it is quite clear that the gravity force between Andromeda and Triangulum should be much stronger than the gravity force between Andromeda and Milky way - due to the distance.

Somehow, it seems that Adromeda is moving away from Triangulum while both of them are moving in the direction of the Milky way.

So, if those two galaxies are so close together, why they are not moving closer to each other?

Why do they prefer to collide with us?

 

Edited April 24, 2016 by David Levy

[Strange]

Because they, and all the other galaxies, are in complex orbits around one another. Right now Andromeda might be getting further from Triangulum but almost certainly, at some time in the past, they were getting closer (until they passed one another).

[David Levy]

How close Andromeda and Triangulum were in the past?

We might be able to verify this question based on the following Hydrogen bridge between the galaxies:

"Two large neighbors of our own Milky Way galaxy—Andromeda (upper right) and Triangulum (lower left)—experienced a close encounter in the distant past."

http://www.sciencemag.org/news/2012/06/scienceshot-hydrogen-bridge-connects-two-galaxies

However, Andromeda is a supper massive spiral galaxy (with about one billion stars), while Triangulum is like a baby spiral galaxy with only 40 M stars.

So, as close they were - as the gravitational power was stronger.

Hence:

- If they were very close to each other in the past, then how could it be that Andromeda didn't destroy Triangulum galaxy?

- Why now they are moving further away from each other – against the gravitational power?

Edited April 24, 2016 by David Levy

[swansont]

Simple question:

 

 

 

!

Moderator Note

 

It doesn't matter if the question is simple or not: if it's not what the thread was discussing, then it doesn't belong there. Starting a new thread is pretty easy, so you need to do that instead of hijacking some other discussion.

 

You need to rid yourself of this bad habit sooner rather than later.

 

 

I was unable to copy over Strange's response: "I suggest you look up the word ORBIT in a dictionary."

 

(IOW, in elliptical orbits, the bodies will spend some of their time moving apart. Your premise is flawed)

[David Levy]

I suggest you look up the word ORBIT in a dictionary.

 

Yes, that is feasible.

However, Orbit path is represented by Newton & Kepler laws. It represents a commitment.

In order to have an orbital path between two astronomical objects, we must verify that the following key requirements are fulfilled:

1. Minimal gravity force

Based on the mass of the two objects and their distance we can easily verify if there is the minimal gravity force which can glue them in an orbital path.

If, for example, we will find a planet at a distance of 10 Mly from us, can we claim that it orbits the sun?

So, we must verify if there is enough gravity force between Andromeda and Milky way to hold the orbital path between them.

In the same token, we must verify if there enough gravity force between Andromeda and Triangulum to hold the orbital path.

 

2. Shape of orbital path

Any orbital path can be cyclic or ellipse.

Based on Newton - In a cyclic orbit, the objects are located at almost the same distance, and their relevant speed is almost constant.

However, those galaxies are moving away (or closer) to each other, therefore, we must look at Kepler law.

Based on Kepler - https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion

The orbit of a planet is an ellipse with the Sun at one of the two foci:

- A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.^[1]

- The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

So, does the path between Andromeda and Milky Way (or the path between Andromeda and Triangulum) is fulfilled one of the above foci?

In other words -

Ellipse path is a fundamental requirement for orbit.

If there is no ellipse path, there is no orbital path.

If they are just moving away (or moving closer) from each other at a constant speed, then this can't be introduced as orbital path.

3. Collision –

In a real orbit path, there is no room for collision!

Two objects in a direct collision, are not at orbital path.

Edited April 27, 2016 by David Levy

[Strange]

As usual, you are making up nonsense instead of attempting to learn. (But at least you didn't end your drivel with "do you agree" so we may be making some progress.)

 

Consider, as simpler example, a comet in orbit around the Sun. It may pass by Jupiter, which is very massive, but it won't suddenly change its course and dive into the surface of Jupiter. It will have its path changed slightly but continue heading away from Jupiter and towards the Sun. It won't dive into the Sun as it passes, it will pass around it and head out to space again.

 

Because its orbit constantly changes because of its encounters with the planets, the orbit will not be a fixed ellipse (so Kepler is irrelevant, except as a short term approximation). And, eventually, the comet may collide with a planet or the Sun.

 

The situation is similar, but much more complex, in the case of galaxy clusters because you have a large number of similarly sized objects in orbit around one another. (So Kepler is almost totally irrelevant.) There paths will be chaotic but they still won't suddenly change course and dive into a collision with a nearby massive galaxy.

 

The only way to model the paths of bodies in a system like this is through simulation. (Not by random guesswork based on a limited understanding of Newtonian gravity.)

1. Minimal gravity force

 

Shown to be an incorrect guess by stars and gas orbiting a black hole or neutron star.

 

 

Ellipse path is a fundamental requirement for orbit.

 

This is only true in the ideal case of one body orbiting another, with no other masses around.

 

Perhaps you could identify the ellipse in these orbits:

 

 

If they are just moving away (or moving closer) from each other at a constant speed, then this can't be introduced as orbital path.

 

Strawman fallacy because no one said that is what is happening.

 

 

In a real orbit path, there is no room for collision!

 

Obviously also false.

 

[J.C.MacSwell]

I don't believe (I am guessing, someone correct me if I am wrong but I am pretty sure it is a good assumption) that we have enough accurate data to know if the path of Triangulum is part of an ellipse or even curved.

 

We can extrapolate backwards using known Laws, and assume that it is.

[Strange]

Correct. All we know are their velocities.

https://en.wikipedia.org/wiki/Velocity_dispersion

[David Levy]

Consider, as simpler example, a comet in orbit around the Sun. It may pass by Jupiter, which is very massive, but it won't suddenly change its course and dive into the surface of Jupiter. It will have its path changed slightly but continue heading away from Jupiter and towards the Sun. It won't dive into the Sun as it passes, it will pass around it and head out to space again.

 

Because its orbit constantly changes because of its encounters with the planets, the orbit will not be a fixed ellipse (so Kepler is irrelevant, except as a short term approximation). And, eventually, the comet may collide with a planet or the Sun.

 

Sorry, it is a severe mistake to use a comet as an example.

The ratio in mass between the Sun and Comet is one to one trillion (and over), while the ratio between Andromeda and Milky way is one to two.

So please, let's use some more realistic example for our case.

 

A ratio of one to two could be represented by a twin star system - Binary star.

 

https://en.wikipedia.org/wiki/Binary_star

"A binary star is a star system consisting of two stars orbiting around their common barycenter."

In Binary star system, the stars do not collide with each other. They are orbiting around their common barycenter.

https://en.wikipedia.org/wiki/Binary_star#/media/File:Orbit5.gif

In the same token, if Andromeda and Milky Way galaxies are in an orbit cycle, then they shouldn't collide with each other. They must orbit around their common barycenter.

​Therefore, comet can't be used as an example for galaxies orbiting system. It is absolutely none relevant example for this case.

Yes, kepler might not be relevant for comet, but it is very relevant for binary star.

 

The situation is similar, but much more complex, in the case of galaxy clusters because you have a large number of similarly sized objects in orbit around one another. (So Kepler is almost totally irrelevant.) There paths will be chaotic but they still won't suddenly change course and dive into a collision with a nearby massive galaxy.

 

You have just offered a solution for: multiple star systems

"Systems of two, three, four, or even more stars are called multiple star systems."

Try to use this system as an example.

 

 

Shown to be an incorrect guess by stars and gas orbiting a black hole or neutron star.

 

How can we compare Andromeda galaxy to Black hole while Milky Way galaxy is compared to star (or gas). That is another severe mistake.

Please try to compare apple to apple.

By using incorrect examples, you are confusing yourself.

Edited April 27, 2016 by David Levy

[Delta1212]

Did anyone suggest that Andromeda and the Milky Way we're orbiting each other? I'm not sure what point you are trying to make, but you seem to be the one who is confused.

[Strange]

Try to use this system as an example.

 

That is exactly what I did.

 

How can we compare Andromeda galaxy to Black hole while Milky Way galaxy is compared to star (or gas). That is another severe mistake.

 

I wasn't making any such comparison.

 

Your inability to understand even very simple English may be the problem here.

[ACG52]

It isn't a language problem. It's a defective thought problem.

[David Levy]

O.K.

 

Two massive galaxies should be compared to binary star system (and absolutely not to Sun/Comet system).

 

That is exactly what I did.

If you have used binary system, how could you claim that Kepler isn't relevant?

Kepler is very relevant - if you are using the term "Orbit"

osted Yesterday, 10:15 AM

Strange, on 24 Apr 2016 - 3:11 PM, said:

I suggest you look up the word ORBIT in a dictionary.

 

Based on your advice, I have looked at the word orbit at a dictionary and it stated:

http://dictionary.cambridge.org/dictionary/english/orbit

"the ​curved ​path through which ​objects in ​space ​move around a ​planet or ​star"

 

So, it is "around a" and not "collide with".

 

Hence, please let's use Kepler or Newton laws and verify if a collision between Andromeda and Milky way could be considered as "orbit".

In the same token and based on those laws, we have to find the real mechanism between Andromeda and Triangulum.

There is no miracle in science. We must base our understanding on pure physics.

Edited April 28, 2016 by David Levy

[Strange]

If you have used binary system, how could you claim that Kepler isn't relevant?

 

I wasn't discussing a binary system, because that is irrelevant to the conditions we are discussing.

 

Kepler is very relevant - if you are using the term "Orbit"

 

Not in the case we are discussing.

 

We must base our understanding on pure physics.

 

Yes. Please do that.

 

How about you produce a simulation of thousands of galaxies orbiting around each other and prove that they never collide.

Edited April 28, 2016 by Strange

[David Levy]

I wasn't discussing a binary system, because that is irrelevant to the conditions we are discussing.

 

So, what kind of system is relevant for you?

 

Do you still insist on Sun/Comet system?

 

Please be aware that the mass ratio is as follow:

 

Comet / Sun = One to over Trillion

 

Milky way / Andromeda = one to two

 

Triangulum / Andromeda = One to 25

 

So based on the above info, would you kindly advice how could we use a Comet /sun system as an example for galaxies?

 

How could it be that with the all examples in the Universe you have selected the most unconnected one?

 

Please try to offer a better realistic example.

 

[Strange]

So, what kind of system is relevant for you?

 

Do you still insist on Sun/Comet system?

 

That was to show that a object can (1) pass by another massive object without falling in (which you claim is impossible) and (2) have its orbit changed from an ellipse so it does collide with the object it is orbiting (which you claim is impossible).

 

Obviously, I was not comparing the mass of the comet to that of a galaxy. However, the point about orbits of multiple bodies is equally true whatever the mass.

 

 

Please try to offer a better realistic example.

 

Please prove that it is impossible for two galaxies to move away from one another in the presence of gravity.

Please prove that it is impossible for two galaxies to collide.

(And incredulity does not count as a mathematical proof.)

[swansont]

Hence, please let's use Kepler or Newton laws and verify if a collision between Andromeda and Milky way could be considered as "orbit".

 

!

Moderator Note

That wasn't proposed, so let's stay on topic. The point was that Triangulum could be orbiting Andromeda, in answer to the question "if those two galaxies are so close together, why they are not moving closer to each other?"

[David Levy]

The point was that Triangulum could be orbiting Andromeda

 

Thanks for the clarification.

 

However, I claim that we have to ask ourselves:

 

What do we really see? Is it orbiting path or escaping path.

 

If the distance between those two galaxies is increasing in a constant speed, than it is escaping by definition. (My opinion)

 

 

In this case, we have to look on the excellent explanation from Strange:

 

 

Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

 

At time 0, they are 1 unit apart:

A.B.C.D.E.F

 

After some time they are 2 units apart:

A..B..C..D..E..F

 

After the same time again, they are 3 units apart:

A...B...C...D...E...F

 

And so on:

A....B....C....D....E....F

 

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

 

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. (But that is OK, because the speed of light limit is a local thing, whereas these objects are in different frames of reference.)

 

So, if A is Andromeda and B is Triangulum

 

 

If it is escaping process, than it should be as follow:

 

At time 0, they are 1 unit apart:

A.B.

 

After some time they are 2 units apart:

A..B

 

After the same time again, they are 3 units apart:

A...B

And so on:

A....B

 

However,

If I understand correctly Strange explantion for "Orbiting", then it should be as follow:

 

 

At time 0, they are 1 unit apart:

A.B.

 

After some time they are 2 units apart:

A..B

 

After the same time again, they are 1 units apart:

A.B

 

After the same time again, they are 0 units apart:

A colide with B.

End.

 

In any case, if we could understand the real mechanism (escaping or orbiting) process, we can get better understanding on the Universe.

Edited April 28, 2016 by David Levy

[Strange]

If the distance between those two galaxies is increasing in a constant speed, than it is escaping by definition. (My opinion)

 

We already know that your opinion on anything to do with science has no value.

 

In this case, we have to look on the excellent explanation from Strange:

 

That is completely irrelevant because, as it clearly says, it is for galaxies that are not gravitationally bound.

 

If I understand correctly Strange explantion for "Orbiting", then it should be as follow:

 

As usual, you understand nothing.

 

In any case, if we could understand the real mechanism (escaping or orbiting) process, we can get better understanding of the Universe.

 

Change the "we" to "I" and you might get somewhere.

 

Do you have any evidence that the galaxies in a cluster are not orbiting one another?

(Your total lack of understanding of physics does not count as evidence.)

[David Levy]

Do you have any evidence that the galaxies in a cluster are not orbiting one another?

(Your total lack of understanding of physics does not count as evidence.)

 

Sure.

 

Orbiting for me is Kepler and Newton.

 

If you can prove that the galaxies are moving to each other based on those laws, then it is orbiting.

 

If galaxies are moving away (or closer) to each other at a constant speed - than isn't "orbiting".

 

Sorry – I just disagree with your explanation about the meaning of "Orbiting".

Edited April 28, 2016 by David Levy

[swansont]

 

Thanks for the clarification.

 

However, I claim that we have to ask ourselves:

 

What do we really see? Is it orbiting path or escaping path.

 

If the distance between those two galaxies is increasing in a constant speed, than it is escaping by definition. (My opinion)

 

 

As has been explained, not only is this factually wrong, it is conceptually wrong to bring an opinion into a discussion using objective facts and definitions. Examples have been given of objects orbiting that have a radial velocity component — they move toward or away from each other — and neither is escaping.

If galaxies are moving away (or closer) to each other at a constant speed - than isn't "orbiting".

 

 

Have you established that the speed is constant? Is it even possible to establish that this is the case, given the short time of our observation?

 

You are making claims that are backed by neither the science nor the data.

[Mordred]

 

Sure.

 

Orbiting for me is Kepler and Newton.

 

If you can prove that the galaxies are moving to each other based on those laws, then it is orbiting.

 

If galaxies are moving away (or closer) to each other at a constant speed - than isn't "orbiting".

 

Why would you think the above if you understood Keplar orbits?

 

Have you be never heard of eliiptical orbits? The speed of the orbitting body is definitely not constant at all points of the orbit.

 

Part of Kepler orbits is the Vis-Viva equation.

 

[latex]v^2=GM(\frac{2}{r}-\frac{1}{a})[/latex]

 

 

https://en.m.wikipedia.org/wiki/Orbital_mechanics

Here perhaps this YouTube will help.

 

https://m.youtube.com/watch?v=6vCl9LHF_8k

Edited April 29, 2016 by Mordred

[pzkpfw]

After the same time again, they are 0 units apart:

A colide with B.

End.

This is quite typical of your technique of latching on to a simplified explanation or analogy, and then picking at it as though that disproves the complete reality.

 

In the real Universe, we have more than one dimension.

 

Sure, if A and B are the only two objects in an empty Universe, and they are "placed" with zero relative motion, they will attract each other and eventually "collide" *.

 

But if you're looking at actual things in the actual Universe, there are many things affecting each other. Mutually attracting objects are not always going to approach each other dead-on. A simple example being comets; approaching from the outer part of the solar system, they swing down past the Sun, can get quite close, but unless they hit it dead on they pass and head back out again.

 

 

* and as Galaxies are not solid objects, even when they do "collide" they don't act like billiard balls; in fact they can pass through each other with interesting effects (and before you nit-pick, no, they won't retain their pre-collision content or shape).

Edited April 29, 2016 by pzkpfw

[David Levy]

Why would you think the above if you understood Keplar orbits?

Have you be never heard of eliiptical orbits? The speed of the orbitting body is definitely not constant at all points of the orbit.

Part of Kepler orbits is the Vis-Viva equation.

[latex]v^2=GM(\frac{2}{r}-\frac{1}{a})[/latex]

https://en.m.wikipedia.org/wiki/Orbital_mechanics

Here perhaps this YouTube will help.

https://m.youtube.com/watch?v=6vCl9LHF_8k

 

Dear Mordred

I fully agree with your description.

However, I disagree with the following statement from Strange:

 

 

Consider, as simpler example, a comet in orbit around the Sun. It may pass by Jupiter, which is very massive, but it won't suddenly change its course and dive into the surface of Jupiter. It will have its path changed slightly but continue heading away from Jupiter and towards the Sun. It won't dive into the Sun as it passes, it will pass around it and head out to space again.

 

Because its orbit constantly changes because of its encounters with the planets, the orbit will not be a fixed ellipse (so Kepler is irrelevant, except as a short term approximation). And, eventually, the comet may collide with a planet or the Sun.

 

The situation is similar, but much more complex, in the case of galaxy clusters because you have a large number of similarly sized objects in orbit around one another. (So Kepler is almost totally irrelevant.) There paths will be chaotic but they still won't suddenly change course and dive into a collision with a nearby massive galaxy.

 

I claim the following:

1. Orbiting - There is no colision between the main objects which are in one orbital system.

As it is stated: "the ​curved ​path through which ​objects in ​space ​move around a ​planet or ​star"

So, it is "around a" and not "collide with".

However, there is a possibility for collision between two objects which are moving at two different orbital systems.

For example, Pluto/Sun and Neptun/Sun systems. Those are two orbital system.

Let's assume that Pluto's orbit cross that of Neptune. If that was the case, then technically those two planets could potentially collide with each other. However, although both of them are orbiting the sun, they are not in the same orbital system. Therefore, a collision is feasible.

In any case, it is not expected that there will be a collision between Pluto and the Sun or Neptune and the Sun.

 

In other words - if Andromeda and Milky way orbiting each other, they shouldn't collide with each other!

 

2. Sun/Comet system - We can't use this example for Andromeda/Milky way (due to mass ratio). We have to use more realistic example as Binary star system (or even Multi Binary stars system).

3. Kepler - Kepler is very relevant in any real orbiting system. If I understand the message from Strange, he claims that Andromeda and Milky way are in orbital path, but kepler is irrelevant for this system. I of course fully disagree with this statement.

4. Better explanation - We need to look for better explanation why Andromeda is moving directly to Milky way (and it is expected that they will collide with each other). In the same token we need to know why Triangulum is moving away (or escaping) from Andromeda.

I claim that Orbital Path can't be a solution for all of that.

 

 

So, what is your advice?

Edited April 29, 2016 by David Levy

[Strange]

1. OK. You insist that the word "orbit" can only be used for a stable 2-body system. Although that is idiotic, let's avoid the word from now on.

 

2. As you reject a simple example (and I can't think of any examples that might be more familiar) you will have to learn the physics rather than just by analogy. (Which we know you are incapable of, so this thread is over.)

 

3. Kepler is only relevant for objects in stable elliptical orbits. We are discussing things which are not moving in simple elliptical paths around a central mass. Therefore Kepler's laws are irrelevant.

 

4. We don't need to do any such thing. You need to learn a little about the physics involved.

 

https://en.wikipedia.org/wiki/N-body_problem

Edited April 29, 2016 by Strange